1. Field of the Invention
The invention relates to methods for synchronizing between a base station and a mobile station in a cell-based mobile communications system.
2. Discussion of the Background
In cell based mobile communications systems, one area of concern is the amount of time and effort required for handover of a mobile station between cells.
In handover, a mobile station's transmission which acts with one cell is switched to another cell (hard handover) or a new cell is added to those servicing the mobile station (soft handover). Generally, the handover is accomplished through a multitude of steps. However, the first step in this multitude is to locate, e.g. to find timing and characteristics of a cell adjacent to a cell or a group of cells currently being used. For this, a procedure commonly known as a cell search is carried out by the mobile station.
A cell search involves the mobile station finding, acquiring, and synchronizing with the tuning and the characteristics and codes of the adjacent or target cell. To facilitate this search, all base stations, each of which services a specific cell, periodically transmit a primary synchronization code or PSC on a primary synchronization channel or P-SCH. All base stations in a communications system transmit the same PSC. Once a mobile station acquires the PSC from a target cell on that target cell's P-SCH, the mobile station uses the PSC to acquire slot synchronization to a cell. It should be noted that slot synchronization is required since, in the time domain, signals are structured into chips, slots, and frames, where one time slot lasts approximately 666 us, a frame lasts for 10 ms or 15 slots and each slot is subdivided into 2560 chips.
Once a mobile station acquires slot synchronization, further steps, such as frame synchronization and scrambling code identification, are carried out. However, these further steps are beyond the scope of this application.
What is of concern is the amount of time and processing power required for mobile stations to acquire a target cell's PSC and P-SCH.
Generally, mobile stations receive signals from possible or candidate target cells while keeping track of already known or active cells. These signals are received, filtered, and correlated with a local copy of the PSC by the mobile station. The mobile station correlates the received complex signal with a local replica of the PSC at all possible time positions over one slot length. The mobile station then sums up, generally non-coherently, a slot profile composed of correlation values for a set number of slots. This is done by summing up the squared amplitude values at specific time positions for every one of the set number of time slots. The mobile station then chooses the largest correlation peak in the slot profile as the time position of the P-SCH of a target cell. It should be noted that the slot profile contains the correlation values for signals which repeat every slot. These signals from the base stations are repeated at approximately the same time position for every slot.
Unfortunately, in most handover situations, the power levels of the signals from active cells and target cells which are received by the mobile station usually have an approximately constant magnitude. Thus, a mobile station may have a slot profile similar to that shown in FIG. 1 where the correlation output of an active cell signal is approximately similar if not greater in magnitude to a correlation output of a target cell's signal. The mobile station which chooses the cell or the cells that correspond to the signals of correlation output in the slot profile that have one of the strongest magnitude can choose the signals of these active cells in its cell search. Since this is not what is desired, the cell search must be restarted, requiring more time and processing power from the mobile station.